1. Field of the Invention
The present invention relates to a sheet feeding device for feeding a sheet which is scanned in a main scanning direction by a light beam, along a feed path having a curved section. Further, the present invention relates to a radiation image reading apparatus which incorporates therein the sheet feeding device.
2. Description of the Related Art
There is known a stimulable phosphor which, when exposed to an applied radiation (X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet radiation, or the like), stores a part of the energy of the radiation, and, when subsequently exposed to applied stimulating rays such as visible light, emits light in proportion to the stored energy of the radiation. A radiation image information recording and reproducing system uses a sheet provided with a layer of the stimulable phosphor, which is referred to as a stimulable phosphor sheet. The radiation image information recording and reproducing system records radiation image information of a subject such as a human body temporarily on the stimulable phosphor sheet, applies stimulating rays such as a laser beam or the like to the stimulable phosphor sheet, photoelectrically reads light emitted from the stimulable phosphor sheet to produce an image signal, and processes the image signal to reproduce the recorded radiation image information on a photosensitive medium such as a photographic film, or display the recorded radiation image information on a display unit such as a CRT or the like.
The radiation image information recording and reproducing system has an image reading apparatus for reading the radiation image information carried by the stimulable phosphor sheet by applying stimulating rays such as a laser beam or the like to the stimulable phosphor sheet which carries the recorded radiation image information in a main scanning direction and feeding the stimulable phosphor sheet in an auxiliary scanning direction that extends substantially perpendicular to the main scanning direction, and an image recording apparatus for recording an image based on the radiation image information on a photographic photosensitive medium by applying a laser beam to the photographic photosensitive medium in the main scanning direction based on the radiation image information obtained from the stimulable phosphor sheet and feeding the photographic photosensitive medium in the auxiliary scanning direction.
Since the medium to be scanned in the image reading apparatus and the image recording apparatus needs to be smoothly fed in the auxiliary scanning direction, the image reading apparatus and the image recording apparatus have a feed mechanism comprising a pair of rollers (nip rollers) rotatable in pressed contact with each other for feeding the medium in the auxiliary scanning direction.
One known feed mechanism of the above type is known as a light beam scanning mechanism as disclosed in Japanese patent publication No. 5-45108. The disclosed light beam scanning mechanism has a pair of rollers for gripping and feeding a medium to be scanned, the rollers being spaced from each other by a gap smaller than the thickness of the medium, and a roller displacing means for displacing at least one of the rollers to widen the gap between the rollers.
The adjustable gap is effective to prevent the medium from suffering shocks when it enters between the rollers. When the medium is not fed by the rollers, the rollers are prevented from being deformed due to the gap present therebetween. The medium to be scanned can thus be fed accurately and smoothly by the rollers in the auxiliary scanning direction.
When the feed mechanism is incorporated in image reading apparatus and image recording apparatus, the feed mechanism is specifically constructed as shown in FIG. 12 of the accompanying drawings, for example. As shown in FIG. 12, the feed mechanism has first and second roller pairs 1, 2 for gripping and feeding a sheet S such as a stimulable phosphor sheet or a photographic photosensitive sheet in an auxiliary scanning direction indicated by the arrow A, and a turn guide 3 extending through the first and second roller pairs 1, 2 for guiding the sheet S along a predetermined feed path.
The first and second roller pairs 1, 2 comprise respective drive rollers 1a, 2a which are driven to rotate about their own axes and respective driven rollers 1b, 2b movable toward and away from the respective drive rollers 1a, 2a. A light beam, such as a laser beam L, for example, is applied to the sheet S between the first and second roller pairs 1, 2 in a direction substantially perpendicular to the auxiliary scanning direction indicated by the arrow A.
The turn guide 3 has curved sections positioned upstream and downstream, respectively, of the first and second roller pairs 1, 2, each of the curved sections having a relatively large radius R of curvature. The turn guide 3 also has straight sections extending from the curved sections toward the first and second roller pairs 1, 2, the straight sections having respective lengths H1, H2. The feed mechanism has a relatively large length H3 in the horizontal direction to provide a space for feeding the sheet S therein.
In recent years, image reading apparatus and image recording apparatus of reduced size have been desired in the art, and there is a demand for a reduction in the space for feeding the sheet S in the feed mechanism, i.e., the length H3. To meet such a demand, it is necessary to reduce the radius R of curvature and also the lengths H1, H2 of the straight sections.
However, since the sheet S such as a stimulable phosphor sheet or a photographic photosensitive sheet is of high rigidity, if the radius R of curvature and the lengths H1, H2 of the straight sections are reduced, then the driven rollers 1b, 2b which are disposed inwardly of the turn guide 3 tend to be lifted out of position due to the rigidity of the sheet S. For this reason, the sheet S may not be fed accurately by the feed mechanism.
One proposal is to position free rollers near the rollers 1b, 2b for preventing the sheet S from being unduly deformed. However, if the sheet S is curled, then the sheet S impinges upon the free rollers, and the quality of the process of reading image information from and recording image information from the sheet S is lowered. The above problem also arises in feeding various sheets such as rigid sheets of paper other than stimulable phosphor sheets or photographic photosensitive sheets.